1. Field of the Invention
This invention relates to a radio communication apparatus such as a mobile telephone, a portable telephone, a cordless telephone, etc., and more particularly to a radio communication apparatus for use in a dual-mode radio communication system, which selectively uses an analog mode and a digital mode to radio a communication signal between a base station and a mobile unit.
2. Description of the Related Art
A dual-mode radio communication system has been proposed as one of movable radio communication systems. The dual mode system means a system which uses both analog mode and digital mode.
The analog mode is a mode where a transmitter apparatus transmits a carrier wave whose frequency, for example, is modulated using an analog speech signal and data, and a receiver apparatus receives and demodulates the frequency modulated wave to reproduce the analog speech signal and the data.
On the other hand, the digital mode is a mode where the transmitter apparatus encodes a speech signal and data and transmits a carrier wave digitally modulated by a .pi./4 shifted DQPSK (.pi./4 shifted Differentially Encoded Quadrature Phase Shift Keying) system with the use of the encoded signal and data, and the receiver apparatus receives and digitally demodulates the modulated wave, thereby decoding the demodulated wave to reproduce the speech signal and data.
Among radio communication apparatuses for use in a dual-mode radio communication system of this type, an apparatus has been proposed which enables a single signal transmission line to be used in both the analog mode and the digital mode by means of a DSP (Digital Signal Processor).
In this proposed apparatus, a modulated carrier wave signal received by an antenna is input to a receiving circuit which is provided with a frequency conversion circuit and a discriminator. The frequency conversion circuit converts the modulated carrier wave signal to an intermediate frequency signal by frequency conversion. The discriminator performs FM demodulation of the intermediate frequency signal received in the analog mode. The signal output from the receiving circuit is input to the DSP via a digital demodulation circuit in both the digital mode and the analog mode.
The digital demodulation circuit performs, in the digital mode, digital demodulation of the received signal to output a digital demodulation signal. On the other hand, in the analog mode, the digital demodulation circuit only converts the received signal to a digital signal, and outputs the digital signal without demodulating the same. In the digital mode, the above-mentioned DSP performs error correction decoding and speech decoding of the digital demodulation signal, and supplies an outputs signal to a PCX decoding circuit. In the analog mode, the DSP supplies the digital reception signal directly to the PCM decoding circuit without decoding the same. The PCM decoding circuit converts the digital reception signal supplied from the DSP to an analog reception signal. The analog reception signal output from the PCX decoding circuit is amplified by a reception signal amplifier, and then output from a speaker.
Since in the apparatus constructed as above, a single signal line is used in both the analog mode and the digital mode, the apparatus can be made more simple in structure and compact than an apparatus which uses two signal lines in the analog mode and the digital mode, respectively.
The apparatus using a single signal line, however, has the following disadvantage:
In the analog mode, FM demodulation by the discriminator causes different reception signal levels in different apparatuses. To compensate for level deviation, it is considered to provide a level-adjusting volume, for example, at the output terminal of the PCX decoding circuit. However, the volume will adversely affect the apparatus in the digital mode. Specifically, since in the digital mode, the discriminator does not perform FM demodulation, the level of the reception signal is substantially constant between different apparatuses. If the volume compensates for the reception signal level in the digital mode in the same manner as in the analog mode, the reception signal level becomes an erroneous level.